This invention relates generally to corrosion resistance treatment of steels and, more particularly, to the corrosion resistance treatment of condensing heat exchanger steel structures exposed to a combustion environment.
Heat exchangers are a key element in many gas furnace applications. Modem high-efficiency gas furnaces typically include a primary heat exchanger and a secondary heat exchanger mounted, in tandem. In the primary heat exchanger, hot combustion products are cooled by extracting heat at a high temperature. The resulting, partially cooled combustion products are then conveyed to the secondary heat exchanger. Typically, such secondary heat exchangers are in the form of a condensing heat exchanger and are used to effect further heat extraction and cooling. In practice, such further heat extraction and cooling commonly results in the condensation of water vapor from the products of combustion and a release of about 10 to 20 percent of the heat otherwise unavailable in the products of combustion. Consequently, furnaces equipped with such condensing heat exchangers can desirably operate at efficiencies in excess of about 88 percent. In fact, typical modem condensing furnaces can achieve AFUE (annual fuel utilization efficiency) ratings of in excess of about 96 percent.
In an effort to enhance the transfer of heat to the circulating air, most condensing heat exchangers employ a fin and tube configuration. Unfortunately, corrosion is a major problem associated with the use of condensing heat exchangers in such gas furnace applications. In particular and as will be appreciated by those skilled in the art, water condensation and evaporation cycles as are typically realized in such applications can lead to undesirable accumulations of salts and low pH conditions within such condensing heat exchangers and thus create or result in a highly aggressive and corrosive conditions within the furnace and, in particular, within or in contact with the condensing heat exchanger. Further, such corrosive conditions are typically further accentuated by the elevated temperatures associated with such combustion environment applications. In practice, such combustion environment temperatures are generally at least about 10-20xc2x0 C. above ambient, with such temperatures generally falling in the range or about 50xc2x0 C. to about 150xc2x0 C.
As will be appreciated, such corrosive conditions and elevated temperatures can undesirably promote corrosion of low cost metal alloy materials that otherwise might find use in such applications. In particular, the presence of nitric and sulfuric oxides can result in the formation of their corresponding acids which can solubilize the otherwise protective surface oxides thus creating a very corrosive environment. Furthermore, condensation-evaporation cycles can lead to an undesirable accumulation of salts on or in the heat transfer tubes of the exchanger such as to result in a breakdown of the protective passivation oxide layer such as may be present on such metal tube surface. In particular, such metal tubes may undergo heavy localized corrosion such as to ultimately lead to xe2x80x9cthrough-wallxe2x80x9d penetration. As will be appreciated, such through-wall penetrations can pose various risks and complications dependent on the particular application. For example, such a through-wall penetration can pose a serious health hazard in residential applications wherein flue gases can mix with hot circulating air.
In view of such risks and complications, various efforts have been made to reduce or minimize the risks associated with or resulting from exposure of heat exchanger metal surfaces to such otherwise corrosive conditions. For example, condensing heat exchangers are commonly manufactured using expensive stainless steels to resist corrosion and provide desirably long life. In addition, various exotic or otherwise relatively expensive metal alloy materials, such as AL-6XN(copyright) and AL 29-4C, each available from Allegheny Ludlum Corporation, Pittsburgh, Pa., have found application in the manufacture or construction of various heat exchanger surfaces, such as heat exchanger tubing, for example, such as occur or may be included in such condensing heat exchangers. Unfortunately, such alloy materials are costly and consequently the manufacturing or production costs of such condensing heat exchangers can be greater than might be desired.
A low cost alternative to exotic and expensive alloys is to use inexpensive alloys, such as 409 SS for example, to which substrate material a corrosion resistant metallic coating has been applied. Various techniques for obtaining a corrosion resistant metallic coating on a substrate have previously been proposed. In general, however, particular coating techniques or methods, precursors, experimental conditions, and apparatus must be carefully chosen depending on the particular desired end product and the expected or anticipated exposure environments or conditions, as well as process, manufacture and production economics.
Identified below are certain such previously disclosed coating techniques. It is critically important to note that, though these previously disclosed coating techniques seek to improve the corrosion resistant of particular substrate materials, they fail to show or suggest the protective coating application onto a substrate metal, such as of ferrous metal, to provide or result in corrosion protection properties to structures formed of such a substrate metal for extended periods of time such as when used in a condensing heat exchanger structure and when disposed in extremely aggressive environments such as a combustion environment involving exposure to combustion products at significantly elevated temperatures.
The diffusion coating of a metal by the simultaneous deposition of Cr and Si onto the metal is taught by U.S. Pat. No. 5,492,727 and related U.S. Pat. No. 5,589,220. The method utilizes a halide-activated cementation pack with a dual halide activator. These patents specifically disclose the codeposition of chromium and silicon and a minor cerium or vanadium content for the coating of a workpiece. These patents further identify and describe resulting workpiece corrosion protection in a chloride and sulfate-containing environment at ambient temperature.
A chemical vapor deposition (CVD) method for case hardening a ferrous metal interior tubular surface by exposure to diffusible boron with or without other diffusible elements such as silicon to enhance the wear, abrasion and corrosion resistance of the tubular surface is taught by U.S. Pat. No. 5,455,068. The use of chemical vapor deposition for deposit of aluminum and a metal oxide on substrates for improved corrosion, oxidation, and erosion protection is taught by U.S. Pat. No. 5,503,874.
A method for producing materials in the form of coatings or powders using a halogen-containing reactant which reacts with a second reactant to form one or more reactive intermediates from which the powder or coating may be formed by disproportionation, decomposition, or reaction is taught by U.S. Pat. No. 5,149,514.
U.S. Pat. No. 4,822,642 teaches a silicon diffusion coating formed in the surface of a metal article by exposing the metal article to a reducing atmosphere followed by treatment in an atmosphere of 1 ppm to 100% by volume silane, with the balance being hydrogen or hydrogen plus inert gas.
A method for depositing a hard metal alloy in which a volatile halide of titanium is reduced off the surface of a substrate and then reacted with a volatile halide of boron, carbon or silicon to effect the deposition on a substrate of an intermediate compound of titanium in a liquid phase is taught by U.S. Pat. No. 4,040,870.
While the methods and resulting coatings disclosed in these patents may improve the corrosion resistance properties of a substrate material coated therewith, even if only for a very short period of time, there is a need and a demand for a protective coating for application onto a substrate metal, such as of ferrous metal, to provide corrosion protection properties to structures formed of such a substrate metal for extended periods of time such as when used in a condensing heat exchanger structure and when disposed in extremely aggressive environments such as a combustion environment involving exposure to combustion products at significantly elevated temperatures.
In view of the above, there is a need and a demand for a corrosion resistant treatment of condensing heat exchanger structures exposed to a combustion environment such as to more freely permit the use of lower cost metals, such as carbon steel and low grade stainless steel, for example, in such applications without incurring the undesired risks or complications associated with corrosion of such lower costs metals.
It is also important to note that corrosion resistance of specific condensing heat exchanger structures for particular combustion environments may require the formation or application of a very specific surface coating or composition onto particular heat exchanger structures or components. Therefore, there is a need for materials and processes that satisfy each requirement for each such environmental condition, particularly in the case of highly corrosive applications such as containing either or both sulfuric and nitric salts or their precursors.
A general object of the invention is to provide an improved corrosion resistant surface composition and treatment of condensing heat exchanger structure metals exposed to a combustion environment.
A more specific objective of the invention is to overcome one or more of the problems described above.
The general object of the invention can be attained, at least in part, through a method for improving the corrosion resistance of a condensing heat exchanger structure comprising a ferrous substrate metal and which structure includes a surface portion at least partially exposed to a combustion environment. In accordance with one preferred embodiment of the invention, such a method involves applying a corrosion resistant diffusion coating onto the ferrous substrate metal via a fluidized bed application.
The prior art generally fails to provide corrosion resistant treatment of condensing heat exchanger structure metals which are exposed to a combustion environment such as to more freely permit the use of lower cost metals, such as carbon steel and low grade stainless steel, for example, in such applications without incurring the undesired risks or complications associated with corrosion in a combustion environment of such lower cost metals. In particular, the prior art generally fails to provide structures and methods which permit the use of low-cost ferrous substrate metals, such as carbon steel and low grade stainless steel, for example.
The invention further comprehends an improvement in a condensing heat exchanger structure in contact with a combustion environment and which structure includes a ferrous substrate metal. In accordance with one preferred embodiment of the invention, such an improved structure includes a corrosion resistant diffusion coating applied to the ferrous substrate metal via a fluidized bed application.
Other objects and advantages will be apparent to those skilled in the art from the following detailed description taken in conjunction with the appended claims and drawings.